1. Field of the Invention
The present invention relates to an apparatus for purifying a liquid effluent containing pollutants, particularly metals and/or radionuclides, as well as to a process for purifying said effluent.
2. Discussion of the Related Art
Numerous industrial processes lead to the formation of effluents containing metals such as e.g. copper or nickel, or heavy metals such as cadmium or mercury. Other processes lead to the formation of radionuclides such as uranium. All these substances constitute a significant pollution source and it is vital to treat them before discharging them into the natural medium.
The metals contained in a liquid effluent are either in the form of ions of the free metal, or are combined in numerous chemical forms. The ionic form of the free metal is generally the most toxic form, whereas the combined form is less toxic. It can even be considered that the toxicity of a metal element decreases with the degree of metal/organic material complexing. The metal is generally complexed with amino acids, polypeptides, fatty acids or polysaccharides. These compounds have the feature of being negatively charged and therefore constitute an anchoring site for metal ions, which are positively charged.
The prior art already discloses a number of processes making it possible to purify an effluent containing metals or radionuclides using bacteria or microalgae.
One of the processes consists of using biofilters, which are obscure reactors filled with rings, to which the bacteria are fixed. The function of these rings is to increase the surface and contact time between the bacteria fixed thereto and the effluent traversing the biofilter. These bacteria can trap certain pollutant elements, or can feed and develop therefrom.
However, if such a process type is not well controlled, it can lead to the replacement of a chemical contamination by a bacterial contamination, which can sometimes be just as dangerous. In addition, the affinity of the bacteria for these metals can lead to the destruction of their walls, so that their fixing properties are lost and they can die relatively rapidly.
Moreover, it has been demonstrated that microalgae are very powerful metal ion absorbers, particularly as a result of their large, negatively charged cellular surface. Certain species, such as e.g. Porphyridium cruentum produce and excrete large amounts of polysaccharides and they can represent up to 50% of the total quantity of organic materials produced by these microalgae. These polysaccharides are polyanionic and have COO.sup.- and SO.sub.3.sup.- sites. This is precisely the characteristic which makes it possible to trap the heavy metals and radionuclides. This is a rapid, reversible phenomenon taking place in a few minutes.
The prior art already discloses a process for the purification of using microalgae and the so-called lagooning method, which consists of culturing in open air tanks a mixture of several microalgal species. The latter develop as a result of nutrient elements and in particular nitrogen supplied by the effluents to be treated. By a photosynthesis reaction, said microalgae produce oxygen, which is then used by the bacteria also located in the lagooning tank. This purification method is mixed, because it makes simultaneous use of the respective metabolisms of the algae and the bacteria, which makes it possible to purify pollutants of various types.
However, this open air procedure suffers from the disadvantage of not making it possible to control or check the selective development of a given microalgal species. It is therefore not possible to favor the specific extraction of a particular metal element or radionuclide.
The prior art also discloses a process industrially used by U.S. companies BIORECOVERY SYSTEM INC. and GEOMICROBIAL TECHNOLOGIES INC. for the accumulation of gold, mercury or uranium. In this procedure, dead cells of microalgae from the Chlorophyceae family (Chlorella vulgaris and Chlorella regularis) are trapped in an opaque substrate in the form of a silica gel column. The assembly constitutes a biofilter. The effluent to be treated passes through the silica gel column and the metal ions are fixed to the cellular walls of the microalgae by electrostatic forces. They occupy the negative sites of these walls, e.g. constituted by carboxylate ions, which are chelating agents. Therefore these microalgae can "bioaccumulate" up to 15% of their dry matter in uranium and up to 10% of their dry matter in gold. The pollutants are desorbed by modifying the pH of the medium containing the microalgae. Under the stress action caused by the pH modification, said microalgae release into the flow the heavy metals and radionuclides which can then be eluted.
According to the inventors of this procedure, it makes it possible to selectively trap certain heavy metals and not ions such as e.g. calcium, which often compete with these metals. Thus, this procedure has better performance characteristics than that using ion exchange resins.
However, this process still suffers from disadvantages. The biofilter can only be used a limited number of times. As the microalgal cells are dead, their walls can only trap a certain quantity of pollutants and when all the chelating agents have trapped a metal ion, it is necessary to recharge the biofilter with new microalgal cells. This leads to long and complex manipulations and a period of inactivity on the part of the biofilter.
An article published in the Compte rendu de l'academie des Sciences de Paris, Jul. 6 1981, series III, pp 35-37 entitled "Production of sulphated polysaccharides by a photobioreactor having immobilized Porphyridium cruentum cells", by C. GUDIN and D. THOMAS, discloses a photobioreactor making it possible to continuously culture living Porphyridium cruentum cells. This culturing takes place under artificial light in the presence of a constant CO.sub.2 supply and in the absence of nitrogen. These cells continuously excrete a soluble sulphated capsular polysaccharide. However, this document provides no possibility for the treatment of a polluted effluent.